The Habitable Zone (HZ) is defined as the range of circumstellar
distances from a star within which a planet could have liquid water on
its surface, given a dense enough atmosphere. This is depicted by the
green annulus in each of the figures in the
Gallery.

The boundaries are calculated using the methodology
of Kopparapu et al. (2013) and
Kopparapu et al. (2014). These boundaries allow
for both
conservative
and optimistic scenarios depending on how
long it is presumed that Venus and Mars were able to retain liquid
water on their surfaces. The conservative model (shown in light green)
uses the ``Runaway Greenhouse'' and ``Maximum Greenhouse'' criteria
for the inner and outer Habitable Zone boundaries respectively. The
optimistic model (shown in dark green) extends the inner/outer
boundaries by using the ``Recent Venus'' and ``Early Mars''
criteria. The Table shows the percentage of
the planetary orbital phase spent in the Habitable Zone for each of
these models.

• How are the number of Habitable Zone
Planets reported on the front page determined?

These are the planets whose orbits lie entirely within the
optimistic Habitable Zone around their respective stars. To locate
these planets, go to the Table then sort by
clicking on the "tHZo (%)" column header
twice. This will bring those planets that spend 100% of their orbital
phase in the optimistic Habitable Zone to the top of the table.

• Where do the stellar/planetary
parameters originate?

The stellar and planetary parameters used to calculate the orbits
and locations of the Habitable Zone are regularly extracted from
the NASA Exoplanet Archive.

The planetary temperature calculations are based upon parameters
that aren't available in all cases, and thus assumptions are
occasionally made. For example, planetary radii are only known for the
transiting planets. In all other cases we apply a power law model as a
first order approximation. We also assume zero albedo such that the
planets absorb all of the incident flux from the host star.

• Are planets in the Habitable Zone
habitable?

This is unknown since there are a variety of planetary properties
that contribute towards habitability as well as the flux received from
the host star. For example, many of the planets described here are
giant planets that may not even have a rocky surface. Moons of those
planets may have conditions more suitable for sustaining life
however.

• Why aren't all planets
listed?

Including planets in the list relies on having both a complete
orbital solution and sufficient information regarding the host star
properties. Since this list is drawn primarily from radial velocity
and transiting planets, it is normally the latter that impedes
inclusion in the list. Also, there can sometimes be a delay from when
new information is published to when it appears in the archives that
this service draws upon. The log of each planets not included in the
list as well as the reason(s) why can be
found here.

• How do I reference this
site?

Links to this site are always welcome. If the site is used for
research leading to a publication, it can be acknowledged as follows:
"This research has made use of the Habitable Zone Gallery at
hzgallery.org." Please also cite the
associated Kane & Gelino article.